Pishkun Reservoir and enclosing dikes are founded on glacial deposits (till) of an outwash plain lying east of the Sawtooth Mountains. The thickness and character of glacial till vary widely over short distances. Till was deposited by advancing ice and runoff fed by melting alpine glacier. The thickness range is suggested by the hummocky topography characterized by rounded hills and undrained or poorly drained depressions. The landscape just west of Pishkun Reservoir is dotted with small lakes and is generally poor drainage. The reservoir and dikes are at the eastern margin of the glacial outwash plain. Locally, erosion has cut through glacial deposits exposing shale and sandy shale of the Two Medicine formation. The thickness range of glacial deposits under and around the reservoir is not well known but may range up to 100 feet.
Pishkun Reservoir is an offstream storage reservoir, about 15 miles northeast of Gibson Dam, and has a capaicity of 46,700 acre-feet. The reservoir is part of the Sun River Project and is formed by eight earthfill dikes with heights ranging from 12 to 50 feet with an overall length of 9,050 feet. The outlet for the reservoir is a 12-foot-diameter, approximately semi-circular, concrete conduit through Dike No. 4. The outlet structure has a maximum capacity of 1,600 cubic feet per second. There is no spillway for the reservoir.